Navigation and locating systems for automotive vehicles have been proposed in which data necessary to determine the location of the vehicle are obtained by planimetric calculations. The actual position is obtained by considering vehicle data from a predetermined initial starting or reference position. It has been found that, as the path of the vehicle increases in length, errors in determining the path may accumulate. It has been proposed to compare the calculated data of the position of the vehicle, that is, calculated based on vehicle operating data with respect to the initial or reference position, and to correct the data from time to time by comparison with data stored in the system based on maps of the area in which the vehicle is operating.
If the vehicle is operating in an urban environment, correction can be effected by comparing the calculated position of the vehicle with respect to the actual position it should have, as determined by a map, and then correcting the position data. When operating the vehicle outside of urban areas, where comparison of data with urban maps is not possible, for example upon cross-country trips, it is no longer feasible to determine the precise position of the vehicle in accordance with a planimetric calculation. Vehicle operating data are derived from wheel sensors and the basis for the impossibility of correcting errors appears to be due to the dynamics of the wheels and the tires with which the wheels are equipped. For example, the diameter of the tires changes upon change in air pressure therein; additionally, the tire diameter may change due to loading of the vehicle, differential loading at different sides of the vehicle, or inclination of the tires upon passing through a curve. For short distances, obtaining vehicle operating data from tires is sufficiently accurate; when long distances are to be covered, however, errors can become cumulative and lead to complete loss of usable data from the vehicle wheels for use in calculating a navigational path or with reference to an actual navigational path to be traversed.
The geomagnetic field provides long distance stability, so that it has been proposed to use the geomagnetic field data rather than vehicle operating data to determine a navigational path. Yet, it has been found that the geomagnetic field is subject to distortion, for example due to large masses of iron, such as other vehicles, especially trucks, bridge structures, railway and urban rail tracks and electrical systems, and electrical currents arising in the form of ground currents or induced from high-tension transmission lines. Distorted earth fields can cause substantial errors, particularly in the determination of a course to be taken over a short distance.
It has previously been proposed-see German Patent Disclosure Document DE-OS 29 27 325, SZCZESNY/TELDIX GmbH,--to evaluate both vehicle operating data and geomagnetic field data and apply both of these data to a navigation or course computer. The course computer or navigation computer receives the sum of prior course data, the difference of the course data, and a correction signal. The correction signal is obtained from geomagnetic field data which have been suitably processed, based on the difference between the actual geomagnetic field and preceding course data. This value is then multiplied with two correction factors. One of the correction factors determines the tracking speed, considering the cycling time of the course computer. The other correction factor is dependent on the moving speed of the vehicle, thus is dependent both on path or course distance as well as on the time taken to cover the distance. If the geomagnetic field is disturbed, the course or navigational direction is determined, essentially, by the vehicle operating data.